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Rossing |
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Namibia |
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U
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The Rossing uranium deposit is located 55 km NW of the town of Swakopmund on the central western Atlantic coast of Namibia, north of Walvis Bay (#Location: ° 28' 57"S, 15° 3' 21"E).
Radium was first discovered in the Namib Desert by Captain Peter Louw in 1928, near the site of the current Rössing mine. The location of the original radium discovery was revisited by the Louw family in the mid 1950s and uranium minerals recognised. Uranium was not the subject of dedicated exploration in the region until the late 1950s, when numerous other uranium occurrences were also located. RTZ conducted a uranium exploration program in the Namib Desert during the 1960s, and in 1966 secured the rights to the Rössing area. After undertaking geophysical and geological investigations, drilling and evaluation located and outlined the low grade Rössing deposit which was amenable to bulk mining. Ten years later, in 1976, the first uranium was produced and Rössing Uranium became Namibia’s first economic uranium mine.
For detail of the regional setting see the separate Husab - Rössing South record.
Mineralisation is hosted by alaskite* bodies (quartz-orthoclase leuco-granite with or without biotite and trace fluorite) which occur within a migmatitic zone. The alaskite exhibits wide textural variations, ranging from aplitic to granitic to pegmatitic and occur as irregular masses and large numbers of dykes/veins which may be discordant or concordant and sometimes gradational with the enclosing metamorphics.
The mineralised alaskites are concentrated near the contact between the Khan and overlying Rossing Formations, both of which belong to the Neoproterozoic Damara Supergroup. The Khan Formation is the uppermost member of the generally arenitic Nosib Group, the oldest Group within the Damara Supergroup, while the Rossing Formation is the local basal unit of the carbonate dominated Swakop Group. In the core of a dome to the north of the orebody the Khan Formation is underlain by the feldspathic quartzites, quartzites, conglomerates and biotitic gneisses of the Etusis Formation constituting the remainder of the Nosib Formation, which in turn rests unconformably on the older pre Damaran Abbabis Complex (a radiometrically anomalous gneissic granite and augen gneiss, dated at 1925 Ma), also represented within the dome. The Khan Formation has less clastics and more calcareous lithologies than the underlying Etusis Formation.
In the mine area the Khan Formation comprises a lower basic gneiss (70 to 150 m thick), followed by a monotonous succession of amphibolitic, pyroxene-hornblende and pyroxene-garnet gneisses (0 to 120m thick), an upper basic gneiss (70 to 100 m thick) with a band of biotite-amphibole schists (10 to 20 m thick) at the top. The overlying Rossing Formation (around 250 m thick) is less extensive than the Khan Formation, is commonly pyritic and consists of a lower marble (20 to 50 m thick which also includes a graphitic variety and interbeds of pyritic and quartzitic schist), biotite-cordierite gneiss (30 to 40 m thick), a conglomerate ( 5 m thick), upper marble (50 to 70 m thick), a further biotite-cordierite gneiss (40 to 50 m thick), and feldspathic quartzite (>100 m thick). The Rossing Formation is unconformably followed by the mixtite/tillites of the Chuos Formation.
A number of granitic types are represented in the area. These include the widespread Salem Granite comprising porphyritic biotite granodiorite/adamellite, which are preferentially found in the cores of synclinal structures and appear to post date the Kuiseb Formation which is a late Swakop Group pelite-quartzite unit with lesser carbonates. A red granite phase closely associated with the Abbabis Complex and the Rossing type 'alaskites', is radiometrically anomalous and comprises gneisses, gneissic granites, leucogranites and pegmatitic granites and is generally found within the Etusis Formation, but occasionally reaches higher levels. Other very late to post tectonic granites occur within the area and have only very low radiometric expressions. The host pegmatitic granites, or alaskites, are also late to post tectonic and are found only in the Central Zone of the Damaran/Katangan Mobile Belt, and are associated with the red granites. These alaskites are found preferentially in and around anticlinal and domal structures cutting the basement, Nosib Group and the Lower Swakop Group and only occur in areas of highest metamorphic grade.
The Alaskite has an initial passive emplacement with the foliation of enclosed blocks paralleling that in the surrounding sequence, followed by substantial pervasive alteration. It is suggested that there were up to seven pulses of alaskite introduction. It is also suggested that the alaskite is a product of migmatisation, with the composition implying derivation from the Abbabis Complex and Etusis Formation. The compositional change from the Khan to the Rossing Formations is believed to have influenced the formation of the uranium mineralisation. Other alaskites common in the area do not host ore.
The migmatisation and early deformation within the host sequence is dated at 665 Ma, followed by the second period of deformation (F3), then the regional granites at 550 Ma and the alaskites at 470 Ma as a late to post tectonic thermal event accompanying the F4 deformation.
The orebody is developed on the SW margin of a structural dome produced by the interference of two fold F3 and F4 structural trends. F3 is also reflected in a major NNE-SSW feature, the Welwitschia Lineament, which is obvious in the topography, geology, magnetics and gravity data and is some 5 to 10 km to the east of the orebody at Rossing.
Uraninite is the dominant uranium mineral with a minor proportion of betafite, and occurs as grains from a few microns to 0.3 mm, averaging 0.05 to 0.1 mm. Monazite is widespread, zircon, apatite and sphene are commonly associated with the radioactive minerals, while trace amounts of pyrite, chalcopyrite, bornite, molybdenite, arsenopyrite and Fe oxides accompany the ore while fluorite is present in a little greater abundance. For more detail and illustrations see Berning (1986).
The original pre-mining reserve was of the order of 200 000 tonnes of U3O8 at an average grade of 0.35 to 0.4 kg/t (0.035 to 0.04%) U3O8.
In 2006, the remaining reserves and resources at Rossing were (Rio Tinto Annual Rept., 2006):
Proved + probable reserves - 132.4 Mt @ 0.032% U3O8 = 42 300 t U3O8.
Measured + indicated + inferred resources - 572.8 Mt @ 0.03% U3O8 = 170 000 t U3O8.
In 2017, the remaining reserves and resources at Rossing were (Rio Tinto Annual Rept., 2017):
Proved + probable reserves - 80 Mt @ 0.040% U3O8 = 19 000 t of recoverable U3O8.
Measured + indicated + inferred resources - 0.7 Mt @ 0.016% U3O8.
Production in 2017 was 2110 t of U3O8
NOTE: Ore Reserves are additional to Mineral Resources.
* Alaskite: A light coloured alkali-feldspar-granite, containing few, if any, mafic minerals, in which quartz constitutes 20% to 60% of the felsic minerals and in which the ratio of alkali feldspar to total feldspar is greater than 90%.
The most recent source geological information used to prepare this decription was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Rossing
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Selected References:
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Basson, I.J. and Greenway, G., 2004 - The Rossing Uranium Deposit: a product of late-kinematic localization of uraniferous granites in the Central Zone of the Damara Orogen, Namibia: in J. of African Earth Sciences v38, pp. 413-435.
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Gray, T., Kinnaird, J., Laberge, J. and Caballero, A., 2021 - Uraniferous Leucogranites in the Rossing Area, Namibia: New Insights from Geologic Mapping and Airborne Hyperspectral Imagery: in Econ. Geol. v.116, pp. 1409-1434.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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